Electroporation is a technique that is used for introducing chemical species into biological cells, and is performed by exposing the cells to an electric potential that traverses the cell membrane. While its mechanism is not fully understood, electroporation is believed to involve the breakdown of the cell membrane lipid bilayer leading to the formation of transient or permanent pores in the membrane that permit the chemical species to enter the cell by diffusion. The electric potential is typically applied in pulses, and whether the pore formation is reversible or irreversible depends on such parameters as the amplitude, length, shape and repetition rate of the pulses, in addition to the type and development stage of the cell. As a method of introducing chemical species into cells, electroporation offers numerous advantages: it is simple to use; it can be used to treat whole populations of cells simultaneously; it can be used to introduce essentially any macromolecule into a cell; it can be used with a wide variety of primary or established cell lines and is particularly effective with certain cell lines; and it can be used on both prokaryotic and eukaryotic cells without major modifications or adaptations to cell type and origin. Electroporation is currently used on cells in suspension or in culture, as well as cells in tissues and organs.
Electroporation is currently performed by placing one or more cells, in suspension or in tissue, between two electrodes connected to a generator that emits pulses of a high-voltage electric field. The pore formation, or permealization, of the membrane occurs at the cell poles, which are the sites on the cell membranes that directly ace the electrodes and thus the sites at which the transmembrane potential is highest. Unfortunately, the degree of permealization occurring in electroporation varies with the cell type and also varies among cells in a given population. Furthermore, since the procedure is performed in large populations of cells whose properties vary among the individual cells in the population, the electroporation conditions can only be selected to address the average qualities of the cell population; the procedure as currently practiced cannot be adapted to the specific characteristics of individual cells. Of particular concern is that under certain conditions, electroporation can induce irreversible pore formation and cell death. A high electric field, for example, may thus produce an increase in transfection efficiency in one portion of a cell population while causing cell death in another. A further problem with known methods of electroporation is that the efficiency of transfection by electroporation can at times be low. In the case of DNA, for example, a large amount of DNA is needed in the surrounding medium to achieve effective transformation of the cell.
Many of the problems identified above are a consequence of the fact that the process of electroporation in both individual cells and tissues cannot be controlled in real time. There are no means at present to ascertain in real time when a cell enters a state of electroporation. As a result, the outcome of an electroporation protocol can only be determined through the eventual consequences of the mass transfer process and its effect on the cell. These occur long after the mass transfer under electroporation has taken place. These and other deficiencies of current methods of electroporation are addressed by the present invention.
Also relevant to the present invention are current techniques for the study and control of mass transfer across cell membranes. Knowledge of mass transfer across cell membranes in nature, both in cells that are functioning normally and in diseased cells, is valuable in the study of certain diseases. In addition, the ability to modify and control mass transfer across cell membranes is a useful tool in conducting research and therapy in modern biotechnology and medicine. The introduction or removal of chemical species such as DNA or proteins from the cell to control the function, physiology, or behavior of the cell provides valuable information regarding both normal and abnormal physiological processes of the cell.
The most common method of effecting and studying mass transfer across a cell membrane is to place the cell in contact with a solution that contains the compound that is to be transported across the membrane, either with or without electroporation. This bulk transfer method does not permit precise control or measurement of the mass transfer across the membrane. The composition of the solution at specific sites is not known and is variable. In addition, when an electric field is present, the local field intensity will vary from one point to another. Furthermore, the surface of the cell that is exposed to the solution is not well defined. Cell surface areas vary among cells in a given population, and this leads to significant differences among the cells in the amount of mass transfer. For these reasons, the amount of mass transfer achieved by bulk transfer processes is not uniform among cells, and the actual amount transferred for any particular cell cannot be determined.
Attempts made so far to overcome the limitations of bulk transfer techniques include techniques for treating individual cells that include either the mechanical injection (microinjection) of chemical compounds through the cell membrane or electroporation with microelectrodes. In injection techniques, the membrane is penetrated with a needle to deliver a chemical agent, localizing the application of the chemical agent to a small region close to the point of injection. This requires manipulation of the cell with the human hand, a technique that is difficult to perform, labor-intensive, and not readily reproducible. Electroporation with microelectrodes suffers these problems as well as the lack of any means to detect the onset of electroporation in an individual cell. These problems are likewise addressed by the present invention.
The present invention arises in part from the discovery that the onset and extent of electroporation in a biological cell can be correlated to changes in the electrical impedance (which term is used herein to mean the ratio of current to voltage) of the biological cell or of a conductive medium that includes the biological cell. An increase in the current-to-voltage ratio across a biological cell occurs when the cell membrane becomes permeable due to pore formation. Likewise, a decrease in the current-to-voltage ratio through a flowing conductive fluid occurs when the fluid draws a biological cell into the region between the electrodes in a flow-through electric cell. Thus, by monitoring the impedance of the biological cell or of an electrolyte solution in which the cell is suspended, one can detect the point in time in which pore formation in the cell membrane occurs, as well as the relative degree of cell membrane permeability due to the pore formation. This information can then be used to establish that a given cell has in fact undergone electroporation, or to control the electroporation process by governing the selection of the voltage magnitude. This discovery is also useful in the simultaneous electroporation of multitudes of cells, since it provides a direct indication of the actual occurrence of electroporation and an indication of the degree of electroporation averaged over the multitude. The discovery is likewise useful in the electroporation of biological tissue (masses of biological cells with contiguous membranes) for the same reasons.
The benefits of this process include a high level of control over the onset and degree of electroporation, together with a more detailed knowledge of the occurrence and degree of permeability created in particular individual cells or cell masses. When applied to individual cells or to a succession of individual cells, this process assures that the individual cells are indeed rendered permeable and are indeed transformed by the introduction of chemical species. The process also offers the ability to increase the efficiency of electroporation by avoiding variations in the electrical environment that would destroy some cells while having an insufficient effect on others.
In some of its more specific embodiments, the present invention involves the use of an electrical cell in which a biological cell can be placed and that contains a barrier that directs the electric current flow and hence the ion flow through a flow path that passes through the biological cell while permitting substantially no electric current to bypass the biological cell. In some of these embodiments, the invention involves the use of an apparatus containing two liquid-retaining chambers separated by a barrier that is substantially impermeable to an electric current. The barrier contains an opening that is smaller than the biological cell such that the biological cell once lodged in the opening will plug or close the opening. To achieve electroporation, the biological cell is secured over the opening by mechanical or chemical means, preferably in a reversible manner so that the biological cell can later be removed without damage to the biological cell. Once the biological cell is secured over the opening, a voltage is imposed between the two chambers and across the biological cell residing in the opening. The passage of current between the chambers is thus restricted to a path passing through the opening and hence through the biological cell. By monitoring the current-voltage relation in the electric cell, the onset of electroporation is detected and the degree of pore formation is controlled, to both assure that electroporation is occurring and to prevent excessive pore formation and cell death. The user is thus afforded a highly precise knowledge and control of the condition of and the flux across the biological cell membrane.
In another series of embodiments, this invention is useful in the diffusive transport of chemical species into or out of a biological cell. In these embodiments, the cell is again divided into two chambers separated by a barrier, and the biological cell is lodged across an opening in the barrier in such a manner that the passage of liquid around the cell from one chamber to the other is substantially prevented. A liquid solution of the species to be introduced into the biological cell is placed in one or both of the chambers. The concentration of the species in the solution differs from that in the cell (either higher or lower, depending on whether one seeks to introduce or remove the species from the cell), or the concentration in one chamber differs from that in the other chamber.
In preferred methods of applying this invention to diffusive transport, the solutions in the two chambers differ in concentration such that the driving force for the diffusive transport is between the two chambers themselves rather than between the chambers and the interior of the biological cell. Knowledge and controlled monitoring of the concentrations in each of the two chambers on a periodic or continuous basis as the diffusion proceeds, together with the precise knowledge of the dimensions of the opening, enables the user to precisely observe and control the rate and amount of the species that enters the cell. The diffusion time can be controlled by imposing stepwise changes in the concentrations in either or both of the chambers, thereby imposing or removing the concentration differential. An application of particular interest is the combination of this type of diffusive transport of a chemical species with controlled electroporation as described in the preceding paragraph.
Each of the various embodiments of this invention may be used with two or more biological cells simultaneously, or cell masses such as in tissue, rather than just one cell. The apparatus described above can be adapted for use with two or more biological cells by arranging the barrier such that the current or diffusive transport will be restricted to a flow path that passes through all of the cells while preventing bypass around the cells. A further application of the concepts of this invention is the electroporation of biological cells suspended in a flowing liquid. Electrodes are placed in fixed positions in the flow channel, and a voltage is imposed between the electrodes while current passing between the electrodes is monitored. Biological cells entering the region between the electrodes will lower the current, the impedance serving as an indication of the presence of one or more cells in the region, and optionally also as a signal to initiate the application of a higher voltage sufficient to achieve electroporation.
Among the advantages that this invention offers relative to the prior art are the ability to treat cells individually and to adapt the treatment conditions to the needs of individual cells. In embodiments where voltage is applied, the monitoring of the impedance affords the user knowledge of the presence or absence of pores and shows the progress of the pore formation and whether irreversible pore formation that might lead to cell death has occurred. An advantage of the barrier-and-opening apparatus is its highly efficient use of electrical energy by virtue of its restriction of the current to a current flow path passing through the opening. A still further advantage is the ability of the apparatus and method to be integrated into an automated system whereby the condition of each cell is monitored by instrumentation and individual cells are lodged in the opening and then removed at times governed by the monitored conditions.
These and further features, advantages and objects of the invention will be better understood from the description that follows.